1. Field of the Invention
This invention relates to a process for removing and recovering arsenic-containing compounds from gases. In particular, this invention relates to a process for removing and recovering organoarsines from gases, in particular, natural gas.
Arsenic-containing compounds in the form of gases have recently been found in natural gas pipelines and in natural gas fields. These compounds have been identified as predominately trimethylarsine (TMA), ethyldimethylarsine, diethylmethylarsine and triethylarsine. Reported concentrations of total arsenic varies from a few ug/m.sup.3 to two mg/m.sup.3. Because of operational problems caused by the formation of corresponding trialkylarsine sulfide residues on many pressure and flow regulation devices in the natural gas distribution system as well as the environmental significance of arsenic, a process for efficient removal of arsenic to reduce the concentration of these types of constituents in natural gas to a level that is acceptable on the basis of present process and toxicological knowledge is required.
2. Description of Prior Art
At the present time, known processes for removal of arsenic from gas streams are costly and/or generally ineffective. One such process is the Parker copper oxide/zinc oxide catalytic process which was originally developed to remove arsenic, in particular inorganic arsine, from the gas stream at high temperature. This process requires that the gas to be treated be preprocessed to remove water and sulfur species because the catalyst is reactive to water at the processing temperature and is also somewhat specific for sulfur resulting in competitive binding of sulfur for the arsenic sites resulting in a probable reduction in catalyst life. Furthermore, the arsenic deposited on the exhausted catalyst cannot be recovered easily, posing an environmental disposal problem.
In addition, several methods for analyzing the amount of arsenic in gas are known. One such method is the use of nitric acid to extract arsenic from natural gas and atomic spectroscopic techniques to determine the concentration of arsenic in the extract. This method provides a means for determining total arsenic in natural gas but is not a suitable field technique because it uses fragile glassware and concentrated nitric acid, the latter being a potential hazard to test personnel.
A second known method for determining the amount of total arsenic in gas is the NIOSH-approved air sampling method which utilizes coconut charcoal tubes to collect arsenic compounds from natural gas and X-ray fluorescence (XRF) and Neutron Activation Analysis (NAA) techniques to determine the amount of arsenic on the charcoal. However, this method is known to produce inconsistent results due to the fact that the efficiency of the charcoal tubes for collection of arsenic compounds from natural gas is substantially reduced due to much higher adsorption affinity of some natural gas components, such as heavier hydrocarbons, to the charcoal surfaces than the arsenic compounds. As a result, the arsenic content of the gas is not fully captured by the charcoal as it passes through the charcoal tubes, resulting in a significant loss of sample. Thus, it is apparent that this method also is not suitable for use in removing and recovering arsenic from gases.
Another known method for determining total arsenic in gas is the peroxydisulfate method for determining total trimethylarsine in which a saturated solution of potassium peroxydisulfate is used to extract the arsenic-containing compounds from a gas containing such compounds. The arsenic can be extracted by static or dynamic methods. The static batch extraction method recovers arsenic quantitatively from the gas by shaking a fixed volume of gas collected at atmospheric pressure with 10 ml. of extractant in a gas collection vessel for more than one hour. The dynamic method uses liquid extraction vessels through which the gas is bubbled through an extraction liquid. However, this method of arsenic recovery is not effective because the recovery of arsenic depends too much on the pH of the extraction liquid and the rate of sparging.